It is, says Terence Conran, all about making. That's what he has loved doing ever since he was inspired by teachers at Bryanston school in Dorset to make pottery and art. He created and endowed the Design Museum in London in 1989 "to encourage this country to become a workshop again"; we "need to encourage people to make things". Its job is "to educate, at all sorts of levels, from schoolchildren to industrialists". Now he is giving the museum the building it currently occupies in Southwark so that it can sell it to help pay for a planned new venue in Kensington. The museum, as is only polite, is honouring Conran's 80th birthday with an exhibition about him, despite his protestations: "I don't want to be seen… I've always kept my name out of the Design Museum."

He delivers almost unchanged the message of the V&A's Britain Can Make It exhibition of 1946, famous in its time, whose note of desperation suggested that maybe, in fact, Britain couldn't or wouldn't make it, as decades of industrial decline subsequently confirmed. Then, he says, there was "a terrible attitude at schools. They would say, 'Johnny, if you don't do your homework you'll end up in a factory' – but what's wrong with a factory?"

Conran has never stopped railing against this attitude. In the 1970s, when Margaret Thatcher was education secretary, he urged her to include design in the curriculum of schools. "To my amazement she agreed." She "had no interest in design. There were Stubbs horsey pictures all over the walls of her office – nothing visually inspiring, unless you were a horse lover. But she had the view that a better educated consumer would boost industry. She was right, but it was the foreign manufacturers that benefited."

Later, as prime minister, she opened the Design Museum. Prince Charles had been invited but it didn't work out after he questioned why the white cuboid museum lacked a pitched roof. Thatcher herself nearly "turned her back on it. She was furious to discover there were foreign products there." Now, even as the limitations of an economy based on financial services are so apparent, Conran is not much closer to being heard. "The message hasn't got through on the making side," he says, much as he might have done in the 1950s.

You wonder whether, after so many decades, he might try a different tack, but Terence Conran hardly comes across as a failure. He sits in a glassy eyrie in Shad Thames, the district of old warehouses near Tower Bridge whose revival he led, chain-smoking large cigars and wearing his invariable blue shirt. He is surrounded by things denoting his interests – a sign saying "Plain Simple and Useful", a steel ruler, a photograph of the Eiffel Tower seen between the legs of a girl. He is slowed a little by recent illnesses but not blunted. Deyan Sudjic, director of the Design Museum, is in attendance, contributing helpful glosses.

And of course Conran is not a failure. He has done enough in his eight decades for several lifetimes. He has had more good meals and good wine, more money and more wives than most men manage. His innovations include the first flat-pack furniture in Britain, the second espresso machine, the promotion of open-plan living, the chicken brick; and, he says, he has "undoubtedly changed the sex life of Europe" by his promotion of the duvet. He has opened shops, restaurants, businesses, written several books and created the Design Museum.

The chicken brick.

He has had a good time along the way, launching enterprises through alliances with friends, lovers and interesting people. He learned how to make black squid risotto from the artist Eduardo Paolozzi – "it was absolutely thrilling" – who also taught him how to cut an onion. In return, Conran showed him how to weld.

He tells how he founded his first restaurant, the Soup Kitchen: "I had an American girlfriend who was financed by her dad rather generously to do Europe. She had an apartment in the Rue Jacob in Paris and found me a job in a restaurant" where "I ended up in the absolutely terrible greasy basement washing pots and pans and saw that the chefs were not exactly trustworthy – they found a way of strapping fillets of beef to their inside leg." So: "I had to find a way of not employing chefs and we had this idea of a huge cauldron that made extremely good stock." All that was then needed was to add different ingredients and sell it at a shilling a pint."

And he created Habitat. After he launched his flatpack furniture ("obviously predating Ikea very considerably") he found that conventional shops had no idea how to present it and sell it. So he decided to "try doing a shop where there is knowledge and enthusiasm" and where his products could be put in the company of others of a similar spirit. Habitat, he believes, changed people's lives: "It was an opportunity to acquire these products that allowed you to lead a contemporary lifestyle, sold at prices that people could afford."

It certainly did the thing with which it is usually credited, which is to play a leading role in the transformation of postwar British taste. Along with Elizabeth David's writing on cookery it opened palates and interiors to international influences. For her it was French cookery; for him it was the legacy of the Bauhaus and modern design as seen in Milan or in the work of the Americans Charles and Ray Eames. "It is hard to overstate how uninteresting London was then," says Conran. "It really was the era of Spam fritters."

Terence Conranat 80: 'He bristles at the suggestion that his greatest innovations have been in the realm of business.' Photograph: Neil Wilder/ John Parkinson Agency

For all his love of making, Conran's success was led by consuming, by a nascent culture of image and selling fuelled by such things as colour magazines. Later he rode the Thatcher boom to his advantage, working property deals and floating Habitat as a public company.

"For the first time in my life I had a load of money," he says and he "wanted to do something meaningful and useful with it", out of which desire came the Design Museum. He bristles at the suggestion that he is not primarily a designer – "like the last president of the United States, I did not know what an entrepreneur was" – but his greatest inventions have been in the realm of business, of buying and selling, rather than in design.

For myself, I find there are limits to the Conran palate and palette. I have never loved a Conran restaurant or a Conran object, for all that I appreciate his considerable contribution to opening up the possibilities of consumption, or like duvets and espresso machines, or recognise the thought and effort that have gone into his creations. They seem a little too managed, manipulated, packaged and don't quite communicate the fun he has got out of life, as if constrained by some invisible boundary.

To help me understand him better his office send me Inspiration, a photographic book of the things he likes best, his "most personal book to date". Here are images of Picasso, and a basket of radishes, and bare-breasted Japanese girls, all arresting, but also levelled, made equivalent and drained of difficulty and content. This levelling makes things easier to sell, but it also makes them less interesting.

It is magnificent that the Design Museum is there at all, and it shows many fine exhibitions, but over the years it has failed fully to capture the abundant energy, diversity and outrageousness of design in a digital, globalised age. I ask Conran if the museum should exhibit things he doesn't like, such as the postmodernism now on show at the V&A, and he pulls a sour face that clearly means "no", but if the museum is to show what is going on, it should be open to everything. The exhibition on Conran will be called The Way We Live Now, but I suspect that much of contemporary life won't be there.

It would take more lifetimes for Conran to do everything that he has done and also be as brilliantly innovative a designer as, say, the Eameses. "What I have done," he says, "is to make things available, in the restaurants and in the shops and in the Design Museum." He's right, but the very best effect of his work to date would be if this making available expanded into worlds he hasn't dreamed of.